The objective of this program project is to establish the micromechanical properties of the individual sensory cells of the organ of Corti in an intact mammalian ear, and to relate the mechanical response of the individual hair cells to a number of other physiological responses measured in the auditory system. An optical sectioning microscope will be developed to visualize the cells in the basal turn of the cat cochlea through an intact round window membrane. A heterodyne interferometer will be developed that will allow measurements of microvibration of single cellular elements inspite of their extremely low optical reflectivity. The cells in the basal turn respond maximally to high auditory frequencies. At these frequencies the method of measurement of acoustical input is not available at present. The definition of the acoustical input at high frequencies will be improved by measuring (i) the acoustic field distribution in the ear canal, (ii) vibration patterns of the tympanic membrane, (iii) impedance of the ear, and by theoretical analysis. Structures such as basilar membrane, tectorial membrane inner and outer hair cells, and supporting cells will be directly visualized and the amplitude and phase of their vibrations will be determined in response to sound pressure near the threshold of hearing. Linear and non-linear properties will be measured. These measurements should allow us to define the relationship between vibrations of the individual cells of the organ of Corti and that of the basilar membrane underneath them. This knowledge will be very valuable in defining the sensory transduction taking place at the inner and outer hair cells. Electrophysiological and histological evaluation will be carried out to monitor the condition of the cells on which measurements will be made.